The present invention relates to surveillance of proper operation of a digital transmission link using pulse regenerator repeaters and to the location of any possible interruption in such a link. It concerns more particularly remote surveillance and fault location units to be situated in the pulse regenerator repeaters of a digital transmission link.
U.S. Pat. No. 3,987,395 describes a known method of remote surveillance and fault location for surveillance of a digital transmission length and for monitoring, at one of the link terminals, the error rate of the set of pulse regenerator repeaters. The method consists in applying a service signal to the link from the opposite end of the link from that at which the error rates are monitored, the service signal being added to the digital data signals conveyed by the link. The service signal is located outside the passband of the pulse regenerator repeaters and is relayed at each pulse regenerator repeater by special circuits referred to as remote surveillance and fault location units. The service signal is constituted by a low frequency carrier modulated by a very low frequency binary signal and is formed by a repetitive frame comprising a synchronising word which is usually generated at the said opposite end of the link from that where the surveillance takes place, followed by time slots in which each remote surveillance and fault locating unit inserts in turn, when it is relaying the service signal around its own pulse repeater regenerator, data significant of the error rates measured at each location along the link. The frame synchronising word may also be generated at a pulse regenerator repeater in the event that the service signal no longer reaches the repeater from further upstream. This makes it possible to locate the break in the link from the end under surveillance by counting the number of occupied time slots in the incomplete frames which arise at said end.
Remote surveillance and pulse location units equipping the pulse regenerator repeaters of such a digital transmission link receive the service signal from the pulse regenerator repeater located immediately upstream therefrom with respect of the direction of transmission towards the end at which surveillance is performed. The units demodulate the service signal to extract the frame, add data representative of the error rates of their own pulses regenerator repeaters thereto and re-transmit the service signal towards the next pulse regenerator repeater situated immediately downstream therefrom. The aim of the present invention is to provide the above-mentioned functions by means of a circuit of simple structure suitable for implementation in the form of an integrated circuit, and which is adapted for use in pulse regenerator repeaters of a digital transmission link.